I'm no expert on GPS, but I do know that there are times of the day when the satellites are in a small group in one part of the sky, not giving a good "fix" (triangulation). In most cases, they are grouped only for a short period of time (usually less than 20 minutes), but when that happens, especially if part of the sky is blocked from view, the GPS cannot get the fix that it needs to find itself.
And sometimes things just don't work....
And sometimes things just don't work....